The invention relates to a top-mounted compensator for use in a motion compensation system like those used in connection with an oil and gas vessel.
The operations of many floating vessels, such as semi-submersible drilling rigs, drill ships, and pipe-laying ships, are impeded by sea swell or waves. Sea waves impart an up-and-down motion to the vessel, commonly referred to as “heave.” The waves can last anywhere from a few seconds up to about 30 seconds or so, and their height can range from a few centimeters or inches up to about 15 meters (about 50 feet) or more.
This up-and-down heave motion is correspondingly imparted to any loads or structures attached to the vessel. Heave motion of the loads or structures is often highly undesirable, and even dangerous, to equipment and personnel. For example, when attempting to drill a wellbore in the sea bed, the heave motion can cause a corresponding motion of the drill string and the drill bit attached to the end of the drill string, severely restricting the operating window of the rig. As much as 20% of rig operating time in the North Sea can be lost “waiting on weather” or calmer seas.
Heave compensation is directed to reducing the effect of this up-and-down heave motion on the load or structure attached to the vessel. “Passive” heave compensation systems fix the load to a point, such as the sea bed. When sea swell causes the vessel to move relative to the load, the passive compensator uses compressed air to provide a low frequency dampening effect between the load and the vessel.
“Active” heave compensation systems measure the movement of the floating vessel using a measuring device such as a motion reference unit and use the measured movement to control a drive that moves a connection device, such as a traveling block or a crane hook, relative to the vessel. The drive that moves the connection device is typically a drawworks (winch) in block and tackle arrangement to the connection device. The drawworks can reel its cable in-and-out to cause the connection device to be raised and lowered relative to the vessel. The block used in the block and tackle arrangement is a crown block.
Prior art crown blocks do not provide for sheave tilt actuation or idler sheave location, make use of a hinge adaptor, and mount the sheave block so that it hangs past or below the crown block yoke, thereby requiring a lot of height, weight and space (see e.g. U.S. Pat. No. 8,534,387 B2 to Taraldrud). As the weight of the crown block increases, so does the weight of the crown beam that supports the block.
Despite the advance in both passive and active heave compensation systems, heave compensation remains a priority to increase the safety and efficiency of floating vessels.